So a couple possibilities come to mind:

1. Someone else has your password. Do you have kids and do they have access to devices which may have your Google account linked? You may want to change your password (use something long, hard to guess and unique).
2. Your local system is compromised in some way. This would be a really odd way for someone to use that access, but it's always possible. Take a look at the apps and any browser extensions you have installed and make sure there isn't anything you don't recognize.
3. There is some sort of Cross Site Scripting (XSS) vulnerability which is being leveraged to subscribe you to stuff. I would expect Google to be better than to have an XSS on YouTube (they bought Mandiant a while ago, FFS). But, big companies doing stupid things is common enough. When you got the pop-up, was it in the YouTube app or a web browser. Did you have other tabs open? Other background processes from sketchy apps?
4. It is Google, them doing shitty things to their product (that's you) for their customers (the advertisers paying for your eyeballs) is basically their business model. Don't like it, de-google your life (warning: this is actually really hard).

It would not surprise me if this is A/B testing a new 'feature'. I would check your account for weird logins though as it sounds like spam trash.

As i recall, the new 'collab' videos or whatever they're called will subscribe to all the channels and not just the one with the video so check any recent videos/channels you've subscribed to.

Do you have carbon monoxide alarms installed in your home?

Only time something like that happened to me was when LTT got hacked. I was suddenly subscribed to some channels promoting a crypto scam pretending to have ties with Elon Musk. Turns out they had hacked and renamed a bunch of channels owned by the Linus Media Group.

Are any of your previous subscriptions missing?

I used to have a similar thing happen on Twitter and my conclusion was that it was simply a channel that I was following that got sold and repurposed for some other use.

I thought maybe I just subscribed to some of the channels I noticed on my subscription list a long ass time ago and forgot, but now I'm questioning that... 🤔

I've never had that happen. Check your account logins to see if your Google account is compromised.

There's a good reason degoogle communities are growing these days. Your best options for YouTube are either getting something like revanced/greyjay/newpipe and watch videos without logging in, but keep in mind that it doesn't count as views for the content creator, and you can also enable SponsorBlock, which supports them even less, so you could support them through patreon or whatever they use. Or you can just abandon YouTube all together, and use one of the many other alternatives like peertube, odyssey, nebula, rumble, bitchute, or whatever else.

This happens most often than you think with YouTube.